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• Why second frame has wrong PTS value when decoding with ffmpeg ?

  21 octobre 2015, par theateist

  I’m using ffmpeg to encode video to mp4.
  I have MP4 that were encoded with different GOP & bitrate values.
  When I decode the video frame by frame the pkt_pts of each successive frame should increase in time base units.
  Some of the MP4s jump in correct successive pts values. For time_base={num=1, den=12800} and frame_rate={num=1, den=25} the interval=512 (12800/25) : first frame has pts=0, second frame has pts=512, third frame has pts=1024,...

  But, there are MP4s that though the frames jump in correct pts values, the SECOND frame has wrong pts value. For time_base={num=1, den=1200000} and frame_rate={num=1, den=16} the interval=75000 (1200000/16) : the first frame has pts=0, the second frame has pts=48000, the third frame has pts=123000.
  Why the second frame has pts=48000 and not pts=75000 ?

  I’m trying to understand what parameters can cause this.

  If you need more information in order to address this question please let me know.

• Trying to stream webcam with VLC, video works but no audio

  19 août 2021, par MirceaKitsune

  I created a bash script a while back to stream my webcam to http with VLC. The line I used was :

  


  cvlc -vvv v4l2:///dev/video0 input_stream --sout '#transcode{vcodec=MJPG,vb=0,scale=1,acodec=none}:standard{access=http,mux=mpjpeg,dst=:1234}'


  


  That works fine but no audio. I've been trying to update it to capture sound from the webcam as well, but nothing I do seems to get that working. I know the ALSA address of the mic which is "hw:2,0" and using the arecord command as a test confirms it works, it's just vlc that isn't capturing or streaming accordingly. Here's the latest command I managed to work out, video is functional but no audio :

  


  cvlc -vvv v4l2:///dev/video0 alsa://hw:2,0 --live-caching=5000 --sout '#transcode{vcodec=h264,vb=1024,acodec=mpga,ab=128,samplerate=44100,channels=2}:standard{access=http,mux=ffmpeg{mux=flv},dst=:1234}'


  


  What is wrong with it and how do you recommend improving this ? Thanks.

  


• saving H.264 encoded images with libavcodec

  8 octobre 2011, par user846400

  I am getting H.264 images from an IP camera and want to save the encoded images (without decoding). I am using output-example.c from ffmpeg (libavformat/output-example.c) for this purpose. For Saving the raw H.264 image, I do the following :

  AVPacket pkt;
  
  av_init_packet(&pkt);
  
  if (c->coded_frame->pts != AV_NOPTS_VALUE)
  
          pkt.pts= av_rescale_q(c->coded_frame->pts, c->time_base, st->time_base);
  
  if(c->coded_frame->key_frame)
  
          pkt.flags |= PKT_FLAG_KEY;
  
  pkt.stream_index= st->index;
  
  pkt.data= (uint8_t *)ulAddr;//video_outbuf;
  
  pkt.size= out_size;
  
  save_image(pkt.data, out_size);

  Where ulAddr is the address pointer to the image and out_size is the image size. Instead of saving the images to a media video file, I want to save the individual images. save_image function simply uses basic fopen and fwrite functions for saving the images. If I decode the frame and then save, everything works fine. But I have problem saving the encoded frames. The encoded frames are saved with a very small size and then they cannot be decoded.
  Is there anything wrong ? I will really appreciate any help in this regard.